Polyisoolefins can be provided as homopolymers or copolymers comprising an isoolefin and a copolymerizable monomer. Butyl rubber is a copolymer of an isoolefin and one or more multi-olefin co-monomers which are usually conjugated dienes. Commercial butyl rubber comprises primarily isobutylene and contains no more than 2.5 mol % of isoprene as the multi-olefin. Butyl rubber is generally prepared using a slurry process in methyl chloride and uses AlCl3 as an initiator. The polymerization is typically carried out at temperatures of −90° C. to −100° C. These low temperatures are required in order to maintain suitably high molecular weights for rubber applications. Raising the temperature or increasing the amount of isoprene above 2.5 mol % results in the formation of a low molecular weight polymer which is no longer suitable for molded article applications.
There are very few techniques known which allow for the incorporation of more than 2.5 mol % of isoprene in butyl rubber. The molecular weight depressing effect of diene comonomers may, in principle, be offset by still lower reaction temperatures. However, in this case secondary reactions that result in gelation tend to occur to a greater extent and such low temperature processes are more costly. Gelation at reaction temperatures of around −120° C. and possible options for the reduction thereof have been described (c.f. W. A. Thaler, D. J. Buckley Sr., Meeting of the Rubber Division, ACS, Cleveland, Ohio, May 6-9, 1975, published in Rubber Chemistry & Technology 49, 960-966 (1976)).
Another technique for the production of >2.5 mol % isoprene involves the incorporation of a second co-monomer which is capable of cross-linking the growing polymer. This technique was used in, for example, Canadian Patent 2 418 884 wherein they used the cross-linking molecule di-vinylbenzene (DVB) to achieve a polymer with >2.5 mol % isoprene and suitably high molecular weights. The use of di-vinylbenzene is problematic as its use in industrial processes is tightly controlled, requiring new techniques for the production of >2.5 mol % butyl rubber.
A substantially gel free polyisoolefin rubber, a butyl copolymer rubber in particular, with a high molecular weight fraction can be made in a “star-branched” configuration. The material is synthesized by the direct copolymerization with a functional reagent (for example, a multifunctional resin) to produce a material with a high MW fraction that is branched. An example of such a rubber is described by Powers et al (U.S. Pat. No. 507,113). The star-branched product produced by this process typically has an average molecular weight of less than 250 g/mol. Powers mentions the prior art related to DVB-modified butyl rubber and characterizes the prior art as deficient, since it relates to polymers having a high gel content in the polymer product. The use of functional reagents to cause branching is undesirable in that it increases the cost and complexity of the polymerization process, and may lead to fouling of the various pipes and reactors.
Canadian patent 1,019,095 [Scherbakova et al. (Scherbakova)] teaches an industrial process for manufacturing butyl rubber in solution. The catalyst system used in the process comprises an alkylaluminum halide (e.g., ethylaluminum sesquichloride ((C2H5)2AlCl—Cl2AlC2H5)), with water or hydrogen sulfide as a co-catalyst, and isopentane as a solvent. The process, most probably takes place at −85° C. to −80° C., with a content of solids in solution at about 10 weight percent. The low solids content makes this process less economic than conventional processes.
There is therefore a need for a polyisoolefin copolymer rubber that has greater than 2.5 mol % multiolefin comonomer has commercially relevant molecular weight characteristics, low gel content, superior dimensional stability, improved processability, low content of environmentally undesirable reagents (such as DVB), is produced at temperatures higher than or equal to those used in current processes and/or is produced at higher solids content for equivalent or improved economics.